The invention relates to the field of friction stir welding (FSW). With this technique elements are joined to one another by means of a mandrel which is rotating or is movable back and forth and which is moved over the boundary surface of the elements. Frictional heat is generated as a result of the interaction between the cyclically moving mandrel and said elements. This frictional heat is dependent on, inter alia, the contact force and the tracking speed of the mandrel.
The characteristic feature of friction stir welding is that no melting of the elements to be joined takes place. A soft, plastic welding zone is formed which is forged under the influence of the cyclic movements of the mandrel.
This joining technique has the advantage that the joint has a high strength which is appreciably higher than the strength obtained with fusion welding such as TIG (tungsten inert gas) and MIG (metal inert gas) welding.
A further advantage is that it is possible to join together alloys and combinations of materials for which the conventional welding techniques are unsuitable. The alloys of the 2000 series (AlCuMg), 6000 series (AlMgSi), and 7000 series (AlZnMgCu) used in the air freight and aerospace industries may be mentioned as an example.
For such materials use must be made of other joining techniques, such as riveting or gluing. However, each of these techniques has drawbacks, such as fatigue problems and relatively high production costs.
EP-B 0 615 480 discloses a method for joining elements by means of friction stir welding. The elements are brought into the plastic state by the moving mandrel, after which hardening commences and the joint is produced.
With friction stir welding, it must be possible to control the heat balance in a desired manner for the purpose of obtaining the plastic state of the elements and maintaining this state for a specific period, which period is needed in order to obtain mixing of the plastic components.
In this respect the invention provides an improved method for joining elements to one another by means of friction stir welding, comprising the following steps:
clamping the elements such that the parts thereof which face one another and are to be joined are immovable with respect to one another,
bringing a mandrel into contact with said parts in the region of the joint to be produced,
cyclic movement of the mandrel with respect to the elements such that frictional heat is generated and the parts are brought into the plastic state,
influencing the distribution of frictional heat in at least one of said elements such that the frictional heat is essentially concentrated in the parts of said at least one element which have been brought into the plastic state, and
allowing the parts brought into the plastic state to cool in order to form the joint.
Especially in the case of metal elements, which have good conductivity in respect of heat and consequently rapidly lose heat, it is important to concentrate the heat as far as possible at the location of the joint. By this means the plastic state can be achieved more rapidly. Moreover, the cyclic movement of the mandrel can be less intensive, whilst nevertheless an appreciable tracking speed remains possible.
As a result the production speed can be higher without an excessively large amount of energy having to be supplied.
The method according to the invention can comprise the step for bringing at least one element into contact with an insulator which has a lower coefficient of thermal conductivity than the elements. Preferably, the major proportion of the surface of the element is brought into contact with the insulator.
In combination with restricting the dissipation of heat in certain regions of the elements, accelerated dissipation of heat in other regions can be desirable. To this end the method according to the invention can comprise the step for bringing at least one element into contact with a heat conductor which has at least the same coefficient of thermal conductivity as said element.
As a consequence of these measures a good resultant joint can be obtained if a rotary mandrel which has a circumferential speed of at most 37 m/min and preferably in the region of 20-31 m/min is used.
The mandrel can be moved relative to the parts with a tracking speed of at least 400 mm/min.
A mandrel which has an external surface made of ceramic material can also be used. The ceramic material can influence the thermal insulation and the generation of frictional heat in a desirable manner.
Such a mandrel can have been coated with a zirconium oxide or with an aluminium oxide.